Electrolytic reduction has been studied for the preparation of uranium metal from uranium oxides. The desired overall reaction is UO.sub.2 + 2C.fwdarw.2CO+U, and the process has many similarities to the Hall-Heroult processes for preparation of aluminum from aluminum oxide using molten cryolite. At temperatures above the melting point of uranium, uranium metal product of good purity was achieved but yields and current efficiencies were very low. Satisfactory feed of UO.sub.2 to electrolytic cells is much more difficult than the feed of Al.sub.2 O.sub.3 to Hall cells. The solubility of UO.sub.2 is about one-tenth that of Al.sub.2 O.sub.3 in Hall cells and the dense UO.sub.2 settles to the bottom where it fouls the metal surface.
Yields and current efficiencies were improved by using compartmental cells to confine the oxide feed and prevent fouling of the metal surfaces. The current efficiencies were 31% as a maximum, and the cell design was not suited to scale-up to large size.
An alternative reduction reaction, UF.sub.4 +C.fwdarw.CF.sub.4 +U, is possible. However, this reaction requires a makeup of UF.sub.4 which is made from the reaction of UO.sub.2 and large amounts of HF, a very costly chemical. Furthermore, CF.sub.4 is a more troublesome gaseous waste than is CO.
Preparation of uranium in electrolytic cells has been successfully demonstrated. One study showed good cell operation using UF.sub.6 as the feed. In another case, a large electrolytic cell was operated with 30 to 50% of the U metal from reduction of UO.sub.2, 50 to 70% from reduction of UF.sub.4. The objective of these studies was to maximize the amount of UO.sub.2 feed; the higher the percentages of UF.sub.4 were easier to use, but a large fraction of UF.sub.4 feed with little UO.sub.2 was not tested. A problem revealed by these studies was that UO.sub.2 settled out in the electrolysis cell due to its low solubility and high density.